The present invention relates to an image forming method, such as electrophotography, electrostatic recording, magnetic recording and toner jetting; and more particularly to an image forming method wherein a toner image is transferred onto a transfer(-receiving) material (recording material) and fixed under heat and pressure to provide a fixed image.
Currently, a printer and a copying machine are required to fulfill high-speed as well as high resolution image formation. For coupling with these requirements, an increased process speed is a subject to be achieved, and particularly matching between a fixing device and a toner in a fixing process (or step) is crucially important.
Further, for such a fixing process, improvements in usability, such as suppression of power consumption and quick start performance are desired.
In such a fixing process, as a fixing apparatus for heat-fixing a toner image (yet-unfixed image) on a recording material, such as a transfer sheet, an electrofax sheet, an electrostatic recording sheet, a transparency sheet (OHP sheet), a printing sheet or format paper, a hot roller-type fixing apparatus has been widely used.
However, a hot roller-type fixing apparatus is accompanied with a problem that the fixing roller has a large heat capacity, so that even if a halogen lamp as a heat source for the fixing apparatus is started to be energized simultaneously with turning on a power supply to the image forming apparatus, it requires a considerable waiting time from a fully cooled-down state of the fixing roller until reaching a prescribed fixable temperature, thus leaving a problem regarding a quick start performance.
Further, even in a stand-by state (non-image forming period), the halogen lamp has to be kept energized so as to maintain a prescribed temperature state of the fixing roller, thus requiring a measure for preventing internal temperature increase in the image forming apparatus and posing a problem of increased power consumption.
For solving the above problem, film heating-type fixing apparatus have been described in, e.g., Japanese Laid-Open Patent Application (JP-A) 63-313182, JP-A 2-157878, JP-A 4-44075, and JP-A 4-204980.
In such a film heating-type fixing apparatus, a heat-resistant film (fixing belt) is inserted between a ceramic heater as a heating member and a pressure roller as a pressing member to form a nip, at which a recording material carrying a yet-unfixed toner image formed thereon is introduced between the film and the pressure roller and sandwiched and conveyed together with the film to supply a heat from the ceramic heater to the yet-unfixed image on the recording material via the film at the nip, thereby heat-fixing the toner image onto the recording material surface also under the action of a pressing force at the nip.
As a characteristic of the film heating-type fixing apparatus, the ceramic heater and the film can be composed of low-heat capacity members to provide an on-demand type device, thus allowing an image forming apparatus wherein the ceramic heater as the heat source is energized to be heated to a prescribed fixing temperature only at the time of image formation, so that the waiting time from the turning-on of the power supply of the image forming apparatus until reaching the image-forming allowable state is short (quick start characteristic) and the power consumption during the stand-by period is remarkably smaller (power economization).
However, the film heating-type fixing apparatus has left a room for improvement when used as a fixing apparatus for a full-color image forming apparatus or a high-speed image forming apparatus requiring a large heat supply. Also, further improvements, regarding improved fixing performance and prevention of difficulties, such as gloss irregularity of fixed images and offsetting, are desired.
As heating means, Japanese Laid-Open Utility Model Application (JP-Y) 51-109739 has disclosed an induction heating-type fixing apparatus wherein a fixing roller is heated with a Joule heat caused by a current passing through the fixing roller induced by application of magnetic flux. According to the proposal, the fixing roller is directly heated by utilizing a generated induction current, thus achieving a higher-efficiency fixing process than a heating-roller-type fixing apparatus using a halogen lamp as a heat source.
However, according to the induction heating roller fixing scheme, a large amount of Joule heat is required for sufficiently heating the roller from room temperature to a fixing temperature, so that it is difficult to shorten the waiting time from the time of power-on to an image forming apparatus to an image formation enabling state, thus achieving the so-called xe2x80x9con-demand fixationxe2x80x9d. Further, as the induction heating roller fixing scheme requires a sufficient preliminary heating of the fixing apparatus, the scheme is not desirable from the viewpoints of obviating temperature elevating in the apparatus and achieving power economization, thus requiring further improvement.
The fixing process generally involves the following problems.
The surface of a heating member, such as a heating roller or a heating film, contacts a toner image in a molten state under a pressure, a portion of the toner image is transferred by attachment onto the heating member surface and re-transferred onto a subsequent fixation sheet, thus soiling the fixation sheet. This is a so-called offset phenomenon, which is largely affected by the fixing speed and fixing temperature. In general, the heating member surface is set at a relatively low temperature in the case of a low-fixing speed, and set at a relatively high temperature in the case of a high fixing speed. This measure is taken to provide a substantially constant heat quantity for toner fixation regardless of a fixing speed.
A toner image on a fixing sheet is formed of a number of toner layers, so that in a fixing system of higher fixing speed thus requiring a higher surface temperature of heating member, there is a tendency of resulting in a larger temperature difference between the uppermost toner layer contacting the heating member and the lowermost toner layer contacting the fixing sheet. As a result, at a higher heating member surface temperature, the uppermost toner layer is liable to cause offset (high-temperature offset), and at a lower temperature, the lowermost toner layer liable to cause offset (low-temperature offset) because of a fixing failure due to insufficient fusion of the lowermost toner layer.
For solving the above problem, it has been generally practiced to elevate the fixing pressure at a higher fixing speed so as to cause anchoring of the toner onto the fixing sheet. According to this measure, it is possible to lower the heating member temperature to some extent and avoid the high-temperature offset of the uppermost toner layer. However, in this case, a very large shearing force acts on the toner, so that the fixing sheet is liable to be wound about the heating member, thus causing winding offset, or a separation claw trace is liable to be left on the resultant fixed image due to a severe action of the separation claw for separation of the fixing sheet from the heating member. Further, because of a higher pressure, the image quality degradation is liable to be cause due to collapse of line images or toner scattering at the time of fixing.
In a high-speed fixing system, a toner having a lower melt viscosity is generally used than in a low-speed fixing system so as to fix the toner image while obviating high-temperature offset and winding offset by lowering the heating member surface temperature and also the fixing pressure. However, when such a toner having a low melt viscosity is used in a low-speed fixing system, the high-temperature offset is liable to be caused.
As a further factor regarding the offset phenomenon, a smaller particle size toner is liable to result in a lower fixability of a halftone image. This is because at a halftone image portion, the toner coverage is low and a small-particle size toner transferred onto cavities on the fixing sheet receives a smaller heat quantity and the toner at the cavities receives also a lower fixing pressure due to obstruction by convexities of the fixing sheet. Further, a toner forming a halftone image and transferred to convexities of the fixing sheet receives a larger shearing force per toner particle because of a smaller toner layer thickness than in a thicker toner layer forming a solid image portion, thus being liable to cause offset and result in a lower quality of fixed image.
In order to solve such problems, it has been practiced to adjust a molecular weight distribution and a crosslinked component amount of a binder resin constituting the toner, so as to be adapted to an objective fixing process.
For example, JP-A 8-262795 has proposed a toner comprising a binder resin characterized by a molecular weight distribution based on gel permeation chromatography including high-molecular weight styrene-acrylic resin having a molecular weight peak in a molecular weight region of at least 5xc3x97105, styrene-acrylic resin having a molecular weight peak in a molecular weight region of 5xc3x97104-5xc3x97105, styrene-acrylic resin having a crosslinked structure and polyester resin having a molecular weight peak in a molecular weight region of at most 5xc3x97104, but the toner has left a room for improvement regarding adaptability to a high-speed fixing system.
Moreover, the fixability of a toner is largely affected by a moisture content of the toner. This is because the moisture content of a toner is instantaneously vaporized at the time of fixation. As a result, at a high moisture content, the toner is liable to be insufficiently melted because a substantial portion of the heat from the fixing apparatus is consumed for vaporization of the moisture, or the fixation of toner is liable to be obstructed by generated steam. The difficulty is pronounced in a fixing system using a low fixing pressure. As a result, it has been desired to develop an image forming method providing high image quality and high fixing performance at the time of high-speed fixation.
JP-A 8-160675 and JP-A 8-202077 have disclosed an improvement in developing performance by adjustment of toner moisture content. However, no reference is made to the influence of moisture content on the fixability and matching with a fixing apparatus.
Further, JP-A 11-249334 has disclosed an influence of residual monomer content on the wax dispersion state to improve the low-temperature fixability. However, no reference is made to the influence of residual monomer content on fixed image quality and matching with a fixing apparatus.
A generic object of the present invention is to provide an image forming method using a dry toner having solved the above-mentioned problems of the prior art.
A more specific object of the present invention is to provide an image forming method including a fixing step showing excellent quick-start performance and power economization characteristic.
Another object of the present invention is to provide an image forming method using a dry toner capable of suppressing offset and exhibiting excellent matching with a fixing apparatus.
A further object of the present invention is to provide an image forming method capable of providing a fixed image of excellent image quality in formation of monotone images, or capable of providing a full-color or multi-color images of excellent quality free from image fixing irregularity.
According to the present invention, there is provided an image forming method, comprising:
heating and pressing a toner image onto a recording material by heat-pressure means to form a fixed image on the recording material, wherein
said heat-pressure means comprises (i) magnetic flux generating means, (ii) a rotatable heating member having a heat generating layer capable of heat generation by electromagnetic induction and a release layer and (iii) a rotatable pressure member forming a fixing nip with the rotatable heating member, so that the toner image on the recording material is fixed under heat and pressure by pressing the rotatable pressure member against the rotatable heating member via the recording material,
the toner image is formed of a toner comprising toner particles each containing at least a binder resin and a colorant,
the toner has a moisture content of at most 3.00 wt. %, and
the toner has a storage modulus at 110xc2x0 C. of Gxe2x80x2 (110xc2x0 C.) and a storage modulus at 140xc2x0 C. of Gxe2x80x2 (140xc2x0 C.) satisfying:
Gxe2x80x2 (110xc2x0 C.)xe2x89xa61.00xc3x97106 dN/m2, and 
Gxe2x80x2 (140xc2x0 C.)xe2x89xa77.00xc3x97103 dN/m2. 
These and other objects, features and advantages of the present invention will become more apparent upon a consideration of the following description of the preferred embodiments of the present invention taken in conjunction with the accompanying drawings.